Method of preparing a mold to form an article and method of forming the article with the mold

ABSTRACT

A method of preparing a mold to form an article is provided. In one embodiment, the method includes preparing a quantity of fluid including lightweight oil, submerging the mold in the quantity of fluid such that the fluid at least partially enters internal cavities of the mold, and filling asperities defined in walls of the mold. The filling includes creating a negative pressure in the mold while submerged in the quantity of fluid, and removing substantially all air from the internal cavities.

BACKGROUND

The field of the present disclosure relates generally to treating porousarticles and, more specifically, to preparing disposable core dies toform a ceramic article.

At least some known turbine components, such as blades, nozzles, andvanes, have complex geometries. For example, turbine blades and nozzlesmay have internal passages and/or voids defined therein that must bemanufactured in accordance with accurate dimensions having tighttolerances. In such instances, investment casting is generally effectiveat manufacturing parts that require precise dimensional accuracy.

When manufacturing turbine components as described herein, investmentcasting may involve forming a disposable core die (DCD) by any suitablemethod. A low-viscosity, silica-based ceramic slurry is typically pouredor injected into the DCD, and the slurry is cured such that the curedceramic slurry conforms to the internal shape of the DCD. The curedceramic core is then fired to produce a solidified ceramic core, thecore is positioned within a shell mold, and the turbine component isformed such that the core defines the internal passages and/or voids ofthe turbine component when the core is removed from the shell mold.

One method of forming a DCD is in a rapid prototyping process. Rapidprototyping involves forming an object, based on a digital model, bylayering material with a 3D printing machine. While rapid prototyping iseffective at creating dimensionally accurate objects, the processgenerally uses auxiliary material to support the object being built. Forexample, solid objects formed with a 3D printing machine may include awax-like auxiliary substance on the outer surface of the object, andhollow objects may also include the wax-like auxiliary substance withininternal cavities of the object. For hollow objects such as a DCD, theauxiliary material may block the passage of slurry therethroughresulting in a ceramic core having incomplete structures.

One method of removing the auxiliary material from the object is to heatthe wax-like substance such that it melts and drips off the object.While this method is effective in removing auxiliary material from theouter surface of objects, heating the wax-like substance may noteffectively remove all the auxiliary material from internal cavities ofhollow objects, especially when the internal cavities have a small andintricate design and/or dead-end passages. Thermal removal may becombined with solvent cleaning to clear excess material from internalcavities of a hollow DCD. While solvent cleaning clears the internalcavities of auxiliary material, a solvent such as hexane generallydissolves at least a portion of the material that the DCD is constructedfrom. As such, open pores may be formed in the previously impermeablewalls of the DCD, which may alter the shape of and result in surfaceroughness on the formed DCD core. Accordingly, when the ceramic slurryis injected into the DCD and solidified, the DCD core formed therefromwill fall outside of acceptable dimensional tolerances because theslurry conforms to the altered DCD shape.

BRIEF DESCRIPTION

In one aspect, a method of preparing a mold to form an article isprovided. The method includes preparing a quantity of fluid includinglightweight oil, submerging the mold in the quantity of fluid such thatthe fluid at least partially enters internal cavities of the mold, andfilling asperities defined in walls of the mold. The filling includescreating a negative pressure in the mold while submerged in the quantityof fluid, and removing substantially all air from the internal cavities.

In another aspect, a method of forming an article with a mold isprovided. The method includes constructing the mold from a firstmaterial, cleaning internal cavities of the mold with a solvent that atleast partially dissolves the first material, filling asperities definedin walls of the mold with a quantity of fluid that includes lightweightoil, wherein the asperities are formed during cleaning with the solvent.The method also includes injecting a second material into the mold,wherein the article is formed from the second material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary container with a quantityof fluid therein.

FIG. 2 is a perspective view of an exemplary oiling assembly for use inthe container as shown in FIG. 1.

FIG. 3 is a perspective view of an exemplary vacuum chamber.

FIG. 4 is a flow diagram of an exemplary method of preparing a mold toform an article.

FIG. 5 is a flow diagram of an exemplary method of forming an articlewith a mold.

DETAILED DESCRIPTION

Embodiments of the present disclosure enable forming a ceramic core in adisposable core die (DCD) with improved dimensional accuracy. Morespecifically, embodiments of the present disclosure facilitate formingsmooth ceramic cores by filling porous die walls with oil. In theexemplary embodiments, a fixtured rack of DCDs are formed in a rapidprototyping process that uses an auxiliary material to support the DCDsduring formation. To remove the auxiliary material from the DCDs, athermal removal and solvent cleaning process is used. The thermalremoval facilitates removing a large portion of the auxiliary materialfrom the DCD, and solvent cleaning is used to dissolve auxiliarymaterial that is not easily removed from internal cavities of the DCDwith thermal removal. In one embodiment, the DCDs are constructed of amaterial that also partially dissolves during solvent cleaning resultingin porous DCD walls. In the exemplary embodiment, the auxiliary materialis a wax-like material, and the DCD material is an organic thermosettingpolymer.

Injection molding using a DCD that has porous die walls may result inthe liquid component of the slurry being absorbed into the walls, whichincreases solids loading in the slurry, increases the slurry viscosity,and may result in non-fill regions within the DCD. To facilitatepreventing these modes of injection failure and to facilitate preventinga ceramic core from conforming to the shape of the porous walls, in oneembodiment the DCDs are treated with a lightweight oil to fill the diewall porosity. In the exemplary embodiments, a solution is formed thatincludes mineral oil diluted in hexane solvent. The DCDs are submergedin the oil-solvent solution such that the internal cavities of the DCDsreceive the solution therein. Because a DCD generally has an intricatestructure with passageways as small as about 10-20 mils in diameter, thesolution may not easily enter such passageways. Accordingly, in theexemplary embodiments, the DCDs are placed in a vacuum chamber whilesubmerged in the solution to remove substantially all of the air fromthe internal cavities. As used herein, the term “internal cavities” mayrefer to internal passageways formed during DCD construction and/orpores defined in the DCD walls formed during solvent cleaning.

It is believed, without being bound by any particular theory, thatsubmerging the DCDs in solution facilitates filling the pores in the DCDwalls with the lightweight oil. The lightweight oil used herein doesn'treact with the ceramic slurry, remains in liquid form and does not dryon the walls of the DCD, and any excess oil that remains on the DCDwalls is forced towards the DCD outlet as slurry is injectedtherethrough. Accordingly, the lightweight oil substantially blocks theceramic slurry from entering the pores to facilitate forming smoothceramic DCD cores that conform to the CAD master design used to form theDCD.

Methods of preparing a mold to form an article, and methods of formingthe article with the mold are described herein. For example, the methodsdescribed herein may include at least one of: a) constructing a DCD(mold) from an organic polymer; b) heating the DCD in a thermal removalprocess to remove auxiliary material from the DCD; c) cleaning internalcavities of the DCD with a solvent that at least partially dissolves theDCD material; d) using compressed air, at about 1.0 bar, to check foradditional auxiliary material blockage in the DCD; e) preparing asolution that includes lightweight oil and liquid hydrocarbon solvent ata weight ratio of about 53:47; f) submerging the DCD in the solutionsuch that the solution at least partially enters internal cavities ofthe DCD; g) creating a negative pressure in the DCD while submerged inthe solution to facilitate removing substantially all of the air fromthe internal cavities and porous DCD walls; h) releasing the negativepressure in the DCD; i) creating the negative pressure and releasing thenegative pressure until substantially all of the air is removed from theinternal cavities; j) removing the DCD from the solution; k) drying theDCD with compressed air blown therethrough at about 1.0 bar; l) dryingthe DCD in a vacuum chamber at a pressure below the vapor pressure ofthe solvent; m) injecting the ceramic slurry into the DCD after removingthe DCD from the solution; and n) curing the ceramic slurry to form asolidified ceramic core.

FIG. 1 is a perspective view of a container 102 with a quantity of fluid104 therein, and FIG. 2 is a perspective view of an oiling assembly 110for use in container 102. In the exemplary embodiment, oiling assembly110 includes a fixture 112 of DCDs 114, and an oiling basket 116.Fixture 112 may be formed by any suitable means, such as a rapid 3Dprototyping process. Once DCDs 114 have been formed and cleaned withthermal removal and solvent cleaning processes, DCDs 114 may beprepared, in an oiling process, to receive ceramic slurry therein. Inthe exemplary embodiment, fixture 112 is placed in basket 116 that issized to be received within container 102. Container 102 has anysuitable quantity of fluid 104 that enables the process to function asdescribed herein. In the exemplary embodiment, container 102 has aquantity of fluid 104 that fills container 102 up to any suitable heightthat enables fixture 112 to be completely submerged therein.

In the exemplary embodiments, the fluid is prepared by dilutinglightweight oil in a quantity of liquid hydrocarbon solvent forming anoil-solvent solution. The solution may include any suitable lightweightoil that enables the solution to function as described herein. As usedherein, the term “lightweight oil” refers to an oil that has a viscosityof less than about 200 centipoise. Further, preferably the oil isnon-reactive with the ceramic slurry, remains in liquid form and doesnot dry on the walls of DCD 114, and may be discharged from DCD 114 whenpresent in excess quantities. An example of suitable lightweight oilincludes, but is not limited to, mineral oil. Further, the solution mayinclude any suitable solvent that enables the solution to function asdescribed herein. An example of a suitable solvent includes, but is notlimited to, hexane. In an alternative embodiment, fluid 104 is alightweight oil, such as silicone oil, undiluted in solvent and having acomparable viscosity to the above described mineral oil-hexane solutionmay be used in place of the mineral oil-hexane solution.

In some embodiments, the weight ratio of lightweight oil to solvent mayvary based on the specific DCD material composition. The lightweight oiland solvent concentrations may be determined based on the amount of diewall open porosity to be filled, the DCD material used, and how much DCDmaterial was dissolved during solvent cleaning. For example, in any ofthe various embodiments of the present disclosure, the lightweight oilconcentration is at least about 25 percent, at least about 30 percent,at least about 40 percent, at least about 50 percent, at least about 60percent, at least about 75 percent, or within a range defined betweenabout 25 percent and about 75 percent by weight based on the weight ofthe solution. In some embodiments, the lightweight oil concentration isabout 53 percent by weight based on the weight of the solution. Further,in any of the various embodiments of the present disclosure, the solventconcentration is at least about 25 percent, at least about 30 percent,at least about 40 percent, at least about 50 percent, at least about 60percent, at least about 75 percent, or within a range defined betweenabout 25 percent and about 75 percent by weight based on the weight ofthe solution. In some embodiments, the solvent concentration is about 47percent by weight based on the weight of the solution.

In any of the various embodiments of the present disclosure, the mineraloil, the mineral oil-hexane solution, and the silicone oil may have anysuitable viscosity that enables the process to function as describedherein. For example, in some embodiments, the mineral oil has aviscosity of about 147 centipoise, the mineral oil-hexane solutionhaving an oil concentration of about 53 percent by weight and a hexaneconcentration of about 47 by weight has a viscosity of about 2centipoise, and the silicone oil has a viscosity of about 5 centipoise.

FIG. 3 is a perspective view of a vacuum chamber 120 used to create anegative pressure within DCDs 114. In the exemplary embodiment, basket116 is moved into position within container 102 such that DCDs 114 aresubmerged in the quantity of fluid 104. Fluid 104 enters internalcavities of DCDs 114, and DCDs 114 remain submerged therein for a periodof time to facilitate filling asperities in the porous DCD walls formedduring the solvent cleaning process. While fluid 104 enters some of theDCD internal cavities at atmospheric pressure, DCD 114 has an intricatestructure with passageways as small as about 10-20 mils in diameter.Accordingly, in the exemplary embodiment, container 102, fluid 104,fixture 112, and basket 116 are positioned within vacuum chamber 120 tofacilitate drawing fluid 104 into DCDs 114, and to facilitate removingall or substantially all of the air that remains within DCDs 114.

In the exemplary embodiment, vacuum chamber 120 is sealed and a negativepressure is created therein. Vacuum chamber 120 may reduce the pressuretherein to any suitable pressure that enables the process to function asdescribed herein. For example, in some embodiments, the negativepressure within vacuum chamber 120 is maintained above the vaporpressure of the liquid hydrocarbon solvent used such that it remains inits liquid phase. In the exemplary embodiment, the vapor pressure ofhexane is about 0.2 bar, and the pressure within vacuum chamber 120 ismaintained at about 0.5 bar.

In some embodiments, the oiling process includes maintaining thenegative pressure within vacuum chamber 120 for a period of time toensure the internal cavities of DCDs 114 are completely filled withfluid 104. In any of the various embodiments of the present disclosure,the negative pressure may be maintained for any suitable period of timethat enables the process to function as described herein. Non-limitingexamples of suitable periods of time include greater than about 1second, greater than about 5 seconds, greater than about 10 seconds,greater than about 30 seconds, greater than about 1 minute, greater thanabout 5 minutes, or within a range defined between about 1 second andabout 5 minutes. In some embodiments, the negative pressure is held forabout 30 seconds. Further, in some embodiments, the negative pressure isreleased and the negative pressure is reapplied to facilitate removingthe air from within DCDs 114. Creating the negative pressure andreleasing the negative pressure may be repeated any suitable number oftimes that facilitates removing air from within DCDs 114.

Once the air has been removed from DCDs 114, the process includessoaking fixture 112 in fluid 104 at atmospheric pressure for a period oftime. In any of the various embodiments of the present disclosure, theperiod of time may be greater than about 5 minutes, greater than about15 minutes, greater than about 30 minutes, greater than about 45minutes, greater than about 60 minutes, greater than about 75 minutes,greater than about 90 minutes, or within a range defined between about 5minutes and about 90 minutes. In one embodiment, fixture 112 soaks influid 104 for a period of about 60 minutes. It is believed, withoutbeing bound by any particular theory, that allowing fixture 112 to soakin fluid 104 facilitates enabling the lightweight oil to uptake into theopen porosity of the DCD walls and remain therein.

Fixture 112 is then removed from fluid 104 and excess solution isallowed to drain from the internal cavities of DCDs and drip back intocontainer 102. DCDs 114 are then dried by blowing compressed airtherethrough to facilitate removing excess solution therefrom, while thelightweight oil remains within the DCD walls. To further dry DCDs 114,fixture 112 may be positioned within vacuum chamber 120 without beingsubmerged in fluid 104. Vacuum chamber 120 is then depressurized to apressure below the vapor pressure of the liquid hydrocarbon solventused. In some embodiments, vacuum chamber 120 is depressurized to a fullvacuum such that any remaining solvent vaporizes and is removed from theinternal cavities of DCDs 114.

FIG. 4 is a flow diagram of a method 200 of preparing a mold to form anarticle, and FIG. 5 is a flow diagram of a method 300 of forming anarticle with a mold. In the exemplary embodiment, method 200 includespreparing 202 a quantity of fluid 104 including lightweight oil,submerging 204 DCDs 114 in the quantity of fluid 104 such that thesolution enters internal cavities of DCDs 114, and filling 206asperities defined in walls of DCDs 114.

Referring to FIG. 5, method 300 of forming a ceramic core (not shown)within DCDs 114 is also described herein. In the exemplary embodiment,method 300 includes constructing 302 DCD 114 from a first material,cleaning 304 internal cavities of DCD 114 with a solvent that at leastpartially dissolves the first material, and filling 306 asperitiesdefined in walls of DCD 114 with a quantity of solution that includeslightweight oil. Once the oiling process has been completed, the methodincludes injecting 308 a second material into DCD 114 such that thesecond material conforms to the shape of DCD 114. In the exemplaryembodiment, the second material is ceramic slurry, and the ceramicslurry is injected into DCD 114 after oiling DCD 114 to fill theasperities formed therein. It is believed that because the lightweightoil remains in liquid form and does not dry on the walls of DCD 114,excess oil that is not absorbed into the porous walls may gravity drainfrom DCD 114. It is also believed that because the lightweight oilremains in liquid form, any excess lightweight oil that remains withinDCDs 114 and is not located in the porous walls will be forced towardsthe outlet of the DCDs 114 and will be discharged therethrough duringslurry injection. As such, the lightweight oil does not adversely affectthe ceramic slurry from being injected into DCDs 114 and taking the formthereof.

Examples

The following non-limiting simulations are provided to furtherillustrate the present disclosure.

Sheets of VisiJet® (“VisiJet” is a registered trademark of 3D Systems,Inc. of Valencia, Calif.) HR200 having a thickness of either 16 mils(0.41 mm) or 40 mils (1.02 mm) were constructed and cleaned with ahexane solvent to facilitate creating open porosity within the samples.Eight samples of 16 mil sheets and eight samples of 40 mil sheets wereformed and cleaned with the hexane solvent. The sheets were subjected totwo hexane solvent cleaning cycles for about 1 hour per cycle at about40° C.

The open porosity percentage of the sheets was determined by usingArchimedean buoyancy measurements in accordance with ASTM C830. The openporosity results are presented in Table 1 below. The samples having a 16mil thickness had an average open porosity of 6.6 percent based on thetotal volume of the samples, and the samples having a 40 mil thicknesshad an average open porosity of 4.6 percent based on the total volume ofthe samples.

TABLE 1 Sample Thickness 16 mil 40 mil Average Open Porosity % 6.6% 4.6%(% total volume of sample)

Accordingly, it has been found that the thicker samples lost lessmaterial, in the form of solubles, when cleaned with hexane. As such,the 16 mil samples had a higher average open porosity percentage thanthe 40 mil samples by about 2 percent.

The eight samples of 16 mil thick sheets, and the eight samples of 40mil thick sheets were solvent cleaned as described above and treatedusing the oiling process as described by embodiments of the presentdisclosure. A solution was prepared by diluting a quantity of mineraloil in hexane. More specifically, a first composition having 20 percentmineral oil and 80 percent hexane by weight was prepared, a secondcomposition having 30 percent mineral oil and 70 percent hexane byweight was prepared, a third composition having 40 percent mineral oiland 60 percent hexane by weight was prepared, and a fourth compositionhaving 50 percent mineral oil and 50 percent hexane by weight wasprepared. Two samples of each respective thickness were submerged in oneof the four compositions and allowed to soak for one hour. The sampleswere soaked, under vacuum, removed from the composition, and dried undervacuum as described above.

The porosity saturation percentage was determined by first calculatingthe open porosity percentage of the samples as described above. Thesamples were then treated with lightweight oil in the process describedabove, dried to remove the hexane solvent such that only the lightweightoil remains, and weighed. The post-treatment weight was compared to thepre-treatment weight of each sample, and the density of the pure oilcomponent remaining therein after hexane removal was determined. The oildensity was used to determine the volume of lightweight oil thatimpregnated the samples, and the determined lightweight oil volume wascompared to the known open porosity volume.

The oiling process results are presented in Table 2 below. The 16 milsamples submerged in the first composition resulted in a 66 percentsaturation of the open porosity, and the 40 mil samples submerged in thefirst composition resulted in a 51 percent saturation of the openporosity. The 16 mil samples submerged in the second compositionresulted in a 110 percent saturation of the open porosity, and the 40mil samples submerged in the second composition resulted in a 94 percentsaturation of the open porosity. The 16 mil samples submerged in thethird composition resulted in a 169 percent saturation of the openporosity, and the 40 mil samples submerged in the third compositionresulted in a 139 percent saturation of the open porosity. The 16 milsamples submerged in the fourth composition resulted in a 173 percentsaturation of the open porosity, and the 40 mil samples submerged in thefourth composition resulted in a 176 percent saturation of the openporosity.

TABLE 2 Sample Thickness 16 mil 40 mil % Oil by Weight % Open PorositySaturation 50 173 176 40 169 139 30 110 94 20 66 51

Accordingly, compositions having higher percentages of lightweight oilby weight more effectively saturated the open porosity of the disposablecore die walls. Further, it was found that loading of 30 percent oil byweight resulted in at least 100 percent open porosity saturation for the16 mil samples, and loading of 40 percent oil by weight resulted in atleast 100 percent open porosity saturation for the 40 mil samples. It isbelieved that saturation of greater than 100 percent does not adverselyaffect the ceramic core forming process because the excess lightweightoil will be removed from the DCD during ceramic slurry injection.

Greater than 100 percent saturation is achieved from solution uptakeinto the samples at the molecular level. As such, saturation of greaterthan 100 percent is used such that the porous walls are substantiallysaturated with lightweight oil after solvent removal with the vacuumdrying process described above. For example, when a sample is oiled witha solution having 50% lightweight oil by weight and 50% solvent byweight, vacuum drying the sample evaporates the solvent and facilitatesreducing the open porosity saturation to about half of the original openporosity percentage. As such, variables such as the percentage of oil byweight in the solution, the open porosity saturation percentage, and theresulting open porosity percentage after solvent removal are taken intoaccount when filling open porosity.

As shown by the experimental results, hexane solvent cleaning results inopen porosity within materials constructed from (organic polymer), andthe oiling process described herein facilitates filling the die wallopen porosity resulting from the solvent cleaning process. As such, theoiling process counteracts the effects of solvent cleaning on DCDs suchthat a rapid 3D prototyping process may be used to create readilyinjectable DCDs that facilitate forming dimensionally accurate moldedparts.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A method of preparing a mold to form an article,said method comprising: the mold is formed from an organic polymer thatat least partially dissolves when cleaned with a solvent, submerging themold in a fluid comprising lightweight oil such that the fluid at leastpartially enters internal cavities of the mold; and filling asperitiesdefined in the walls of the mold, wherein the asperities are formedduring the cleaning with the solvent, said filling comprising: creatinga negative pressure in the mold while submerged in the fluid; andremoving substantially all air from the internal cavities.
 2. The methodin accordance with claim 1, wherein the fluid comprises a solution thatincludes the lightweight oil and a solvent.
 3. The method in accordancewith claim 2, wherein the lightweight oil is at least about 25 percentby weight of the solution.
 4. The method in accordance with claim 2,wherein creating the negative pressure comprises maintaining thenegative pressure above the vapor pressure of the solvent.
 5. The methodin accordance with claim 4, wherein creating a negative pressurecomprises maintaining the negative pressure at about 0.5 bar.
 6. Themethod in accordance with claim 2 further comprising: removing the moldfrom the solution; and drying the mold in a vacuum chamber at a pressurebelow the vapor pressure of the solvent.
 7. The method in accordancewith claim 1, wherein creating the negative pressure further comprisesmaintaining the negative pressure for a period of about 30 seconds.
 8. Amethod of preparing a mold to form an article, said method comprising:the mold is formed from an organic polymer that at least partiallydissolves when cleaned with a solvent, submerging the mold in a fluidcomprising lightweight oil such that the fluid at least partially entersinternal cavities of the mold; filling asperities defined in the wallsof the mold, wherein the asperities are formed during the cleaning withthe solvent, said filling comprising: creating a negative pressure inthe mold while submerged in the fluid; and removing substantially allair from the internal cavities; and releasing the negative pressure; andrepeating creating the negative pressure and releasing the negativepressure until substantially all of the air is removed from the internalcavities.
 9. The method in accordance with claim 1, wherein creating thenegative pressure comprises placing the mold and the fluid in a vacuumchamber.
 10. The method in accordance with claim 2, wherein the solventis a liquid hydrocarbon.
 11. The method in accordance with claim 10,wherein the solvent is hexane.
 12. The method in accordance with claim3, wherein the solvent is at least about 25 percent by weight based onthe weight of the solution.
 13. The method in accordance with claim 2,wherein the lightweight oil is about 53 percent by weight of thesolution and the solvent is about 47 percent by weight of the solution.14. The method in accordance with claim 1, wherein the lightweight oilis mineral oil.
 15. The method in accordance with claim 1, wherein thelightweight oil is silicone oil.
 16. The method in accordance with claim2, wherein the lightweight oil is between about 25 to about 75 percentby weight of the solution and the solvent is between about 25 to about75 percent by weight of the solution.
 17. The method in accordance withclaim 1, further comprising soaking, the mold in the fluid atatmospheric pressure for at least about 5 minutes.
 18. The method inaccordance with claim 17, further comprising removing excess fluid fromthe mold by blowing compressed air through the mold.